Regioselectivity and regiospecificity

Note that the terms regioselective and regiospecific, however, are defined in terms of each other. A regiospecific reaction is one that is 100% regioselective. 

The general terms regioselective and regiospecific have been introduced to describe addition reactions that proceed selectively or exclusively in one direction with unsymmetrical alkenes. Markownikoff s rule then describes a general case of regioselectivity that operates because of the stabilizing effect of alkyl and aryl groups on carbonium ion centers. 

Further disadvantage of the alkyne-azide cycloaddition is the lack of regiospecificity. On the other hand, cycloadditions of azides to alkenes are, in most cases, regioselective and afford 1,5-disubstituted triazolines . Therefore, the regioselective cycloaddition of an azide to an alkene, followed by aromatization (see Section 4.01.5.3.1) is an alternative method for the synthesis of 1,2,3-triazoles. 

Sauers and coworkers have applied the Paterno-Biichi reaction to engeneral formula 427 (Scheme XXXIV) Reductive cleavage of these products with lithium aluminium hydride is also regioselective and leads, following oxidation, to ketones... 

Reactions in which one product predominates are termed regioselective and those in which one is formed exclusively are regiospecific. The electrophilic addition reactions in this chapter are... 

In specifying selectivity of ligogenic processes, the focus of attention is on either whole molecules, or, molecular sites. Selectivity for whole molecules is based cn their morphic relationships and is termed morphoselectivity. In contrast, if the emphasis is on selective transformations at molecular sites, then one is dealing with situselectivityP The latter term is synonymous with site selectivity, site specificity, positional selectivity, locoselectivity, atroposelectivity, regioselectivity (intramolecular), regiospecificity, chemoselectivity (intramolecular), intramolecular selectivity, fimctional selectivity, and sequence selectivity. This chapter deals with the concept of situselectivity, and establishes the commonality of all the previous terms. 

There is considerable current interest in the use of microemulsions as media for lipase-catalyzed stereo- and regiospecific ester synthesis. In some cases lipases seem to display a higher degree of regioselectivity in microemulsions than in aqueous solution. 

Syntheses of nor-mahanimbine and its regioisomers have been effected (65) by reacting 2-hydroxycarbazole (142) with citral (148) in boiling pyridine showing that the reaction is regioselective, not regiospecific. 

The photochemical addition of azirines to the carbonyl group of aldehydes, ketones, and esters is also completely regiospecific (77H(6)143). Besides the formation of the isomeric oxazolines (50) from (39) and ethyl cyanoformate, there is also formed the imidazole (51) from addition to C=N in the expected regioselective manner. Thioesters lead to thiazolines (52), while isocyanates and ketenes produce heterocycles (53). 

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